3 files changed, 27 insertions, 4 deletions

diff --git a/doc/misc/ChangeLog b/doc/misc/ChangeLog
index 40c1743e56b..41559f85cb5 100644
--- a/doc/misc/ChangeLog
+++ b/doc/misc/ChangeLog
@@ -1,3 +1,9 @@
+2009-07-21  Jay Belanger  <jay.p.belanger@gmail.com>
+
+        * calc.texi (Undoing Mistakes): Mention that the undo list will be
+        truncated when Calc is quit.
+        (Customizing Calc): Document `calc-undo-length'.
+
 2009-07-20  Chong Yidong  <cyd@stupidchicken.com>
 
         * calc.texi (About This Manual): Don't mention chapter numbers in text.
diff --git a/doc/misc/calc.texi b/doc/misc/calc.texi
index 0475ee27fc6..0e6fc6db8d8 100644
--- a/doc/misc/calc.texi
+++ b/doc/misc/calc.texi
@@ -10280,10 +10280,13 @@ are removed or restored.  If it was a ``store'' operation, you are
 queried whether or not to restore the variable to its original value.
 The @kbd{U} key may be pressed any number of times to undo successively
 farther back in time; with a numeric prefix argument it undoes a
-specified number of operations.  The undo history is cleared only by the
-@kbd{q} (@code{calc-quit}) command.  (Recall that @kbd{C-x * c} is
-synonymous with @code{calc-quit} while inside the Calculator; this
-also clears the undo history.)
+specified number of operations.  When the Calculator is quit, as with
+the @kbd{q} (@code{calc-quit}) command, the undo history will be
+truncated to the length of the customizable variable
+@code{calc-undo-length} (@pxref{Customizing Calc}), which by default
+is @expr{100}. (Recall that @kbd{C-x * c} is synonymous with
+@code{calc-quit} while inside the Calculator; this also truncates the
+undo history.)
 
 Currently the mode-setting commands (like @code{calc-precision}) are not
 undoable.  You can undo past a point where you changed a mode, but you
@@ -35047,6 +35050,15 @@ as @samp{a/(b*c)}. If @code{calc-multiplication-has-precedence} is
 of @code{calc-multiplication-has-precedence} is @code{t}.
 @end defvar
 
+@defvar calc-undo-length
+The variable @code{calc-undo-length} determines the number of undo
+steps that Calc will keep track of when @code{calc-quit} is called.
+If @code{calc-undo-length} is a non-negative integer, then this is the
+number of undo steps that will be preserved; if
+@code{calc-undo-length} has any other value, then all undo steps will
+be preserved.  The default value of @code{calc-undo-length} is @expr{100}.
+@end defvar
+
 @node Reporting Bugs, Summary, Customizing Calc, Top
 @appendix Reporting Bugs
 
diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 77dd24db798..6d480f8d7d3 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2009-07-21  Jay Belanger  <jay.p.belanger@gmail.com>
+
+        * calc/calc.el (calc-undo-length): New variable.
+        (calc-quit): Truncate rather than eliminate `calc-undo-list'.
+
 2009-07-21  Richard Stallman  <rms@gnu.org>
 
         * files.el (auto-save-mode): Handle buffer-save-size = -2